This invention relates generally to battery protection circuits. More specifically, this invention relates to thermal series protection circuits for rechargeable batteries.
Batteries, when placed in electronic devices like cellular phones, discharge at a controlled rate. For example, a lithium ion battery in a cellular phone may supply current, or discharge, at a rate of about half an ampere. This is analogous to pouring milk out of a jug at a nice, steady, even pace. The reason for this controlled discharge is that the phone""s circuitry serves as a load on the battery, and uses the energy stored in the battery slowly. The discharge rate goes down as the impedance of the load goes up. The high impedance of the phone, and thus the slow energy usage, keeps the current drain limited to a controlled amount.
Occasionally, however, the positive and negative terminals of a battery can become shorted, which means that there is a load with very small impedance on the battery. When the impedance is very small, the current drain is very high. Imagine, for instance, carrying a battery in your pocket. If a piece of conductive metal, like a keychain, bracelet, paper clip, comes into contact with both the positive and negative terminals of the battery, a xe2x80x9cshort circuitxe2x80x9d, or very low impedance load, is created across the battery. When this occurs, the battery discharges at a very high current. The current is higher than is specified for the battery and thus cell life, performance, and reliability rapidly degrades. This battery killing situation is known as the xe2x80x9ckey chainxe2x80x9d problem.
Rechargeable batteries can be expensive. As battery designers want to keep consumers happy, they want to prevent key chain problems from killing batteries, thereby saving consumers"" money. Consequently, designers add xe2x80x9cshort circuit protection circuitsxe2x80x9d to rechargeable battery packs. These short circuit protection circuits are able to sense the level of current draining from the battery. When the current level gets higher than a specified limit, the short circuit protection circuit disconnects the battery from the external terminals.
A common device used to prevent a battery from discharging at high currents is a fuse. When a high current passes through a fuse, the conducting element in the fuse xe2x80x9cbums upxe2x80x9d or clears. For example, when three amperes flow through a one amp fuse, the fuse element burns and the fuse opens. In a battery pack this clearing serves to disconnect the battery from the terminals. The problem with a fuse, however, is that once the fuse clears, it can not be reset. Thus the battery is dead and can not be brought back to life.
Another common device used in short circuit protection circuits, which can reset itself, is a positive temperature coefficient, or PTC, device. A polymeric PTC is a device that protects circuits by going from a low impedance (high current) to a high impedance (low current) state in response to heat. A PTC is essentially two pieces of metal with a matrix of crystalline organic polymer containing conductive elements sandwiched in between. A PTC resembles a square Oreo(copyright) cookie, with metal plates as the cookie halves and crystalline polymer as the tasty cream filling. The active element in a PIC is the polymer, and it takes the form of a malleable goo much like the filling in an Oreo(copyright). Under normal conditions, current flows from one cookie through the filling to the other cookie. Under short circuit conditions however, the high current flowing through the PTC causes the device to heat, which in turn causes the filling of the cookie to go into a high impedance state, thereby blocking current and effectively disconnecting the battery cell from the external terminals.
There are several problems associated with PTC devices when used as short circuit protection elements in battery packs. First and foremost, PTC devices are discrete parts that are large relative to other circuit components in battery packs. For example, a typical surface mounted PTC measures 0.374xc3x970.264 inches. This can be fifty to a hundred times larger than other components. Designers like to keep battery circuits small, so large parts that take up a lot of room tend to make battery packs bigger.
Second, there are manufacturing problems associated with PTC devices. As stated above, a PTC is like an Oreo(copyright), with metal plates as cookies and the polymer as the filling. In order for the PTC to work properly, the two metal plates, or cookies, must remain separated by the polymer, or filling. If a person or machine pinches the two plates together, the PTC becomes a short and no longer functions as a protection device. This occurs often in manufacturing.
Third, over-current protection circuit using PTC devices can not prevent high current discharge quickly. For example, in a short circuit protection circuit using a PTC device, some time is required for the device to heat causing the impedance to increase. Therefore, the abnormally high current continues to flow while the impedance of the device is increasing. In effect, damage may actually occur to the battery even though a PTC short circuit protection device is in place.
Due to the limitations of fuses and PTC devices listed above, there is a need for an improved short circuit protection device that can be manufactured in rechargeable battery packs.